4,4'-diisocyanato dicyclohexylmethane ("PICM") is a cycloaliphatic diisocyanate of low volatility. PICM and other aliphatic isocyanates are useful in the preparation of non-discoloring polyurethanes. In general, such isocyanates are reacted with glycols and/or polyols and chain extenders and/or cross linkers. Such isocyanates are particularly useful in the preparation of polyurethane coatings and elastomers. PICM and the diamine precursor, 4,4'-diaminodicyclohexylmethane ("PACM"), exist in three stereoisomeric forms (i.e., trans,trans; cis,trans; and cis,cis) as described, for example, in U.S. Pat. Nos. 2,606,925, and 3,789,032, Canadian Patents 961,049 and 971,184, and British Patent 1,220,715. Commercial grades of PACM normally contain all three isomers. The most direct method of producing PICM is to first hydrogenate diamino diphenylmethane to form a mixture of the stereoisomers of PACM, and to then phosgenate the mixture. When the synthesis of PICM is conducted using readily available mixtures of stereoisomers of PACM (such as the equilibrium mixture described in U.S. Pat. No. 3,155,724), the PICM obtained is a slurry at normal operating temperatures, having a melting point of about 58.degree. C., which corresponds to a trans,trans-isomer content of about 54%. Various PICM mixtures are known in the art which have trans,trans-isomer contents of from about 18 to about 55% by weight. In addition, the art has recognized an advantage in utilizing high trans,trans-isomer PICM in producing elastomers (see, U.S. Pat. No. 3,789,032). In order to prepare PICM of relatively high trans,trans-isomer content, the art has generally used a PACM having a relatively high trans,trans-isomer content in the phosgenation reaction. Various methods are known for treating PACM to obtain the requisite high trans,trans-isomer content. Crystallization techniques have been described in the art. See, e.g., U.S. Pat. Nos. 2,494,563, 3,153,088, 3,384,661 and 3,393,236. The crystallization of PACM suffers from various disadvantages. PACM readily forms a precipitant when exposed to carbon dioxide, causing problems in filtering and contamination of the crystals (see, U.S. Pat. No. 2,494,563, column 3, lines 26-29, and column 4, lines 72-75). In addition, PACM is generally difficult to crystallize since it will easily form a supercooled liquid. The prior art has overcome this problem by adding seed crystals (U.S. Pat. No. 2,494,563), by lowering the viscosity by using an inert solvent (U.S. Pat. Nos. 2,494,563, 3,153,088, 3,393,236 and 3,384,661), or by forming an adduct of PACM that crystallizes better, such as the hydrate (U.S. Pat. No. 3,153,088) or the alcoholate (U.S. Pat. No. 3,384,661). Such an adduct must be treated to remove water or alcohol before phosgenating to PICM.
It is known to separate the trans,trans-isomer from an industrial mixtures of PICM having a trans,trans content of from 18 to 24% by weight by fractional crystallization followed by washing with cold hexane and then vacuum distillation. The mixture was cooled to 10.degree. C. until crystallization took place and the solid fraction (the trans,trans-isomer) was removed by filtration in a nitrogen atmosphere. The residue was washed with cold hexane and stored under a nitrogen atmosphere. The filtrate was recooled and any new solids removed. See, Byrne et al, "A Study of Aliphatic Polyurethane Elastomers Prepared From Diisocyanate Isomer Mixtures," Rubber Chemistry and Technology, Vol. 58, 1985, pages 985-996, and Wong et al, "Structure-Property Relationships of Transparent Aliphatic Polyurethane Elastomers From the Geometric Isomers of Methylene bis(4-Cyclohexyl Isocyanate)," Advances in Urethane Science and Technology, Vol. 9, 1984, pages 77-101. The major disadvantage of this method is that solvent is required. In addition, the yield of the trans,trans-isomer is poor. Finally, cooling below room temperature is expensive on an industrial scale.